In order to simplify this disclosure by uniquely identifying important parts related to a typical machine system, which may be a system for mechanical quality control, we shall in the following explain the use of some general terms. The term “machine” denotes any production or quality control machine, such as machining centers for milling, drilling, turning, etc., an EDM (Electrical Discharge Machine), a CMM (Coordinate Measuring Machine), a touch probe and stylus position sensing machine, a computer vision system, even a simple mechanical support structure, or similar. “Work piece” denotes a part to be machined or to be subjected to quality control. The actual area on the work piece that has been machined, or quality controlled, is denoted “work area”. The part or device that is performing the actual machining or quality control of the work area is denoted “work tool”. The work tool can be a machining tool (for milling, turning, drilling, etc.), a spark erosion tool (EDM tool), a touch probe or stylus position sensor, an optical imaging sensor, an electromagnetic sensor, or similar. Typically, the work piece is fastened to the machine by use of a “work holder”. We shall denote the following 3 distinct parts of the work holder by “work holder support”, “work clamp”, and “work locator(s)”. In some cases, the work holder, or some of its parts, may be identified as an integral part of the machine.
In relation to the definition of present day mechanical reference systems, the work locators play a crucial role. These work locators are defined mechanical positions against which the work piece is placed, and thereby the work piece position is defined relative to the machine and work tool. All the mechanical parts of a machine including the different support structures, work pieces, work tools, and work holders we shall call a machine part.
The position of the work piece, or the different parts of a machine, etc., shall, unless otherwise stated, in this document typically mean the position, orientation, or both of the aforementioned, relative to another part. We shall call the meeting position between the work tool and work piece “the work position”.
There are no viable techniques available for reading the work position directly at the time when the machining or quality control actually is taking place. In many cases the positional precision of machines relative to the work locators are taken care of by automatically reading the exact position from e.g. glass rulers, by indicating the position of the corresponding translation stages, by relying on motor position controllers and encoders, or simply by use of fixed mountings. In order to position the work piece, the work holders either simply clamp it against some more or less prepared locator surfaces, or clamp it against well-prepared mechanical locator pins. Then, the work piece position is found by use of a position sensor. In some cases work holders themselves also contain position sensors. However, ultimately what is required is to accurately know the work tool position(s) in relation to the work area position(s). Those positions are not readily available for direct reading since the positions readers are located offset from the work area and work tool positions. This means that quite often residual angular and translation offsets, between the reading and working positions, are not properly taken care of. In addition actual work piece clamping, work tool replacement, moving a work piece from one machine to the other, and mechanical transmission errors, may result in unaccountable offsets between work tool and work area positions. Finally unaccountable displacement errors also occur during machining, or a quality control sensor reading, as can be the case when thermal and mechanical forces are working, or the work tool is changed, changing position, wearing down, or replaced. In general, these errors are tackled by relying on good craftsmanship in making, or using, work holders with properly arranged work locators and holding forces. For reference, see “Fundamentals of tool design”; John G. NEE, ISBN: 0-87263-490-6, 1998, 769 pp (Publisher: Society of Manufacturing Engineers, Dearborn, Mich. 48121). However, regular updating of a machine with correct position values during the work process is an elaborate task that is seldom undertaken, and when an update is performed important reference data may not be sufficiently accurate, or they may be lacking all together. A good example is when a chuck mounted milling cutter is replaced with a chuck mounted stylus sensor in order to change the process from machining to position quality control, and back again. This may introduce an unknown position offset between the machining and position control tool that does not make it possible to correctly deal with offset position errors.
To improve accuracy, or counteract the fact that unaccountable displacements may occur, several techniques are in common use such as: mechanical touch probe and stylus sensing, macroscope and microscope viewing, laser beam obstruction sensing, and pressure transducer sensing, see “Modern Machine Shop's Handbook for the Metalworking Industries”; Editor Woodrow Chapman, ISBN: 1-56990-345-X; 2002, 2368 pages, (Publisher: Hanser Gardner) and “Modern Machine Shops Guide to Machining Operations”; Woodrow Chapman; ISBN: 1-56990-357-3, 2004, 968 pages (Publisher: Hanser Gardner). The touch probe determines the position of the work piece by use of a position sensing stylus tip. This touch probe can be mounted on the work tool carrier, in the work tool chuck, or possibly on the work piece carrier. In order to work properly it must either be calibrated each time it is used, permanently fastened, or reproducibly mounted in a mechanical fixture or chuck. During machining the touch probe must be removed, otherwise it may collide with the work holder or work piece. In order to accurately find 2D or 3D positions, or angular displacements, several somewhat time consuming readings in different directions have to be carried out. Alternatively, through a combination of microscope viewing and position control of the microscope, key relative work piece and work area positions can also be found, or calibrated. But, since human viewing is involved, this technique lacks the potential of becoming fully automated, and also has difficulty in finding work holder locator positions. Yet other techniques help determine the position of the work tool tip by reading the degree of obstruction of a laser beam, or reading the work tool tip pressure by use of a pressure transducer. During wear this position can be updated. But these techniques do not tell where the work piece is placed.
Like microscope viewing, techniques related to manual, semi-automated, or automated camera viewing should be useful in finding relative or calibrated positions on the work piece and work area. U.S. Pat. No. 6,782,596 B2 discloses yet another approach where a plurality of datum are disposed and calibrated relative to a work piece before the work piece is disposed in a machine. However, according to the knowledge of the present inventors, common to all presently known techniques is that they lack the ability to do regular position sensing while machining or quality control is taking place, and/or that they depend on calibration and position sensing external to the machine itself. They also lack a holistic approach that properly copes with the position errors of other parts than those actually machined or controlled. During the work processes they are relatively time consuming in their operation, and may in some cases themselves introduce unaccountable positional errors.